You have to copy one of these dlls “libpng12d.dll” “libpng12.dll” “libpng.dll” “libpng13d.dll” “libpng13.dll” to your emacs-23.1/bin/ directory. They require zlib1.dll which you have to copy as well. I did the same thing for jpeg62.dll and giflib4.dll and now my emacs supports jpg, gif and png files. For some reason it does not work if I simply put these dlls in the path.
You can check (image-type-available-p 'png) to see if png is supported. image-library-alist maps image type to a list of dlls which support it.